An image reading device having an auto document feeder (hereinafter, simply referred to as “ADF”) and a flat bed scanner (hereinafter, simply referred to as “FBS”) is known. The image reading device includes a close-contact line sensor such as a contact image sensor (CIS) reciprocating relative to a platen glass. A document placed on the platen glass is optically read by the line sensor while the line sensor moves along the platen glass. The document placed on a document bed of the ADF is optically read by the line sensor disposed at a predetermined reading position while the document is being conveyed along a conveying passage.
When a dot document such as a printing matter is read by the above-mentioned line sensor, an interference pattern called a moiré may be generated. In order to suppress the generation of the moiré, JP-A-2002-171391 discloses a configuration for changing a relative distance between a reading surface of a document placed on the platen glass and a line sensor by vertically moving the line sensor.
In the image reading device described in JP-A-2002-171391, an image reading unit including a charge coupled device (CCD) and a rod lens array is mounted on a carriage. The image reading unit includes an actuator in which an oval disc cam is disposed on a rotation shaft of a stepping motor. The image reading unit is mounted on a carriage in a state where the circumferential surface of the disc cam is in contact with the carriage. By driving the stepping motor, the disc cam rotates and thus a distance between the image reading unit and the carriage is changed. As a result, a distance between the image reading unit and the platen glass is changed.
In JP-A-2002-171391, it is described that a modulation transfer function (MTF) value is measured on the basis of an image signal obtained by reading a focus detecting chart and the height of the image reading unit is changed on the basis of the MTF value. The image reading device has two reading modes of a photograph mode and a printing matter mode. In the photograph mode, the image reading unit is disposed at a height where the MTF value is the maximum and a document on the platen glass is read. In the printing matter mode, the image reading unit is disposed at a height where the MTF value is about 40% of the maximum value and a document on the platen glass is read.
Similarly to the above-described image reading device, JP-A-2002-262032 discloses the relative distance is adjusted by changing the height of the line sensor. The image reading device described in JP-A-2002-262032 includes a rail of which the longitudinal direction is a moving direction of the CIS. The CIS moves over the rail along the rail. A cross-section of the rail in a direction perpendicular to the longitudinal direction (axis direction) thereof is oval and the rail rotates around the axial direction by means of an operation of a lever, thereby serving as a disc cam. The CIS gets close to or apart from the platen glass with the rotation of the rail. That is, the relative distance between the CIS and the reading surface of a document on the platen glass is changed by means of the operation of the lever. JP-A-2002-262032 also discloses that a jack is disposed between the CIS and a supporting plate for mounting the CIS, and the CIS vertically moves by the use of the jack by operating a knob.
JP-A-9-65078 discloses that a moiré preventing plate is used to read a document in which a moiré occurs. The moiré preventing plate is a transparent member on which a document is placed, and is disposed on the platen glass. The document in which the moiré occurs is placed on the moiré preventing plate disposed on the platen glass. In this way, by interposing the moiré preventing plate between the platen glass and the document, the distance between the line sensor and the document increases. Since the document is read by the line sensor in an out-of-focus state, the occurrence of a moiré is suppressed.
In the image reading device described in JP-A-2002-171391, a driving mechanism for vertically moving the line sensor is required to change the relative distance between the reading surface of the document and the line sensor. This is true in using the configuration disclosed in JP-A-2002-262032. Accordingly, the configuration becomes complicated, thereby increasing the cost for manufacturing the image reading device.
In the image reading device described in JP-A-9-65078, an operation of disposing the moiré preventing plate is required to change the relative distance between the reading surface of the document and the line sensor, thereby reducing the occurrence of a moiré. Such an operation is troublesome for a user.
A configuration for changing the relative distance between a reading surface of a document conveyed by the ADF and the line sensor is not disclosed in any patent document. In addition, a configuration for changing the relative distance in the course of reading a document is not disclosed in any patent document.